The difference between high-voltage and low-voltage wiring harnesses is that high-voltage terminals carry a larger current, which easily generates heat, leading to a decrease in the mechanical strength of the terminals and the insulation performance of the wiring harness. At the same time, it causes conductor oxidation, which further aggravates the problem of heat generation.
High-voltage terminal crimping requires consideration of both reliability and low temperature rise at the crimping point. This article primarily discusses the impact of cold crimping on terminal temperature rise.
Common crimping types of high voltage terminals
Crimping, resistance welding, high-frequency welding.
Crimping uses crimping equipment and dies to connect wires and terminals together through a crimping process. High-frequency welding uses a high-frequency welding machine to weld wires and terminals together. Resistance welding uses specialized resistance welding equipment to connect wires and terminals together.
Advantages of conventional crimping: simple operation, convenient use and maintenance, low manufacturing cost, high operating efficiency, and suitable for mass production. Disadvantages: cannot meet the requirements for wire harnesses and terminals that demand high current carrying capacity and low resistance after connection.
The common crimping method for large-square high-voltage terminals is shown in Figure 1, which is a hexagonal enclosed terminal.
The advantages of conventional crimping are obvious, but it is particularly important to maximize these advantages and minimize the disadvantages by reducing crimp resistance. Reduced resistance means reduced heat generation, which lowers product temperature rise and improves product lifespan and quality.
Hazards of terminal overheating
When terminals heat up, they and the contact surfaces of the conductors are prone to oxidation, forming a thin oxide film. This increases contact resistance, and the rate of increase multiplies with rising temperature, further accelerating the terminal's temperature rise and potentially causing a fire. Simultaneously, it anneals the elastic elements of the contact structure, reducing contact pressure and further exacerbating the increase in contact resistance. Furthermore, heat causes the insulation layer of the connecting wires to age and become brittle, leading to decreased insulation performance and posing a risk of leakage, overheating, and fire.
Three major heat sources in the terminals
Conductors: Conductors themselves have resistance. The smaller the cross-sectional area, the higher the resistance. Resistance will cause heat to be generated.
Terminal crimping: Insufficient compression ratio will cause the conductor to loosen, resulting in higher resistance and easy overheating. Over-crimping can easily cause the cross-sectional area to decrease, resulting in insufficient current carrying capacity and overheating.
At the male and female terminals mating point: poor terminal contact or oxidation of the terminal contact surface leads to severe overheating.
Methods to reduce terminal temperature rise
Reduce contact resistance:
Use materials with low resistivity. Commonly used high-voltage terminals are H62 or H65 copper or high-conductivity copper. For products exceeding 125 A, it is recommended to use high-conductivity copper with low resistivity. Reduce conductor contact resistance. Press the terminals firmly against the conductor to reduce crimping resistance. Increase the conductor's cross-sectional area to reduce wire temperature rise.
Increase the heat dissipation area of the conductor:
Forced cooling can be employed, using methods such as air cooling or water cooling. Conductors should be arranged strategically; high-current wiring harnesses should be placed in easily accessible spaces to facilitate natural heat dissipation.
Impact of crimping on temperature rise
The crimping was performed in accordance with the voltage drop test requirements in section 4.2.6 of QC/T 29106—2014 "Technical Conditions for Automotive Wiring Harnesses" and the temperature rise test requirements in GB/T 20234.1-2015 "Connecting Devices for Conductive Charging of Electric Vehicles Part 1: General Requirements". The process is shown in Figures 3 and 4, and the data obtained are shown in Table 1.
Compression ratio/compression efficiency calculation method
(1) Refer to VW60330—2013 standard
(2) Refer to SAE/USCAR21-2014 standard
(3) The difference between conductor compression ratio and terminal compression rate
According to the VW60330-2013 standard, the compression ratio calculation only includes the conductor without terminal compression, which can more intuitively reflect whether there are gaps in the conductor. When the compression ratio is ≤100%, there should be no gaps. We can call it the conductor compression ratio.
According to the SAE/USCAR21-2014 standard, the calculation of the compression ratio includes the compression of the conductor and the terminal. Although it cannot directly reflect whether there is a gap in the conductor, it can more directly reflect the actual cross-sectional area of the crimp joint. For ease of data comparison, this paper defines the terminal compression ratio as 100T/(At+Ac).
Both calculation methods have their advantages.
Analysis results
01
According to Table 1, when the conductor compression ratio of product #3 reaches 104%, the tensile strength has reached the standard of 50mm² and conductor tensile strength ≥2700N specified in QC/T29106-2014. However, the crimping joint is not completely compacted at this time, which poses a significant safety hazard. Therefore, tensile strength cannot be used as a standard for judging the quality of high-voltage terminals.
02
The resistance values in the table do not perfectly correspond to the temperature rise trend, likely due to fluctuations caused by differences in individual terminal contact resistance and inconsistencies in terminal plating oxidation. However, the general trend generally follows the relationship that lower resistance corresponds to lower temperature.
03
The compression ratio of terminal #7 is 73%. Both the conductor and the terminal surface will have an oxide layer. The oxide layer will be gradually destroyed as the compression ratio decreases. When the terminal compression ratio is 73%, the conductor oxide layer begins to collapse, making the copper wires fuse more tightly and the temperature rise also decreases slightly, indicating that the terminal compression ratio is appropriate at this time.
04
Judging from the temperature rise fluctuation, crimping can affect the temperature rise by up to 10°C. This has a relatively large impact on high-voltage terminals.
05
With a terminal compression ratio of 60% for product #10, the theoretical cross-sectional area of the conductor at the crimped terminal is only 30 mm². According to SAE/USCAR21-2014 terminal compression ratio calculations, the cross-sectional area at the crimped terminal should include both the conductor and the terminal's cross-sectional area. In reality, the sum of the conductor and terminal's cross-sectional areas after crimping is 66.97 mm², which is greater than the nominal conductor cross-sectional area of 50 mm². Therefore, this compression ratio will not cause the crimped area to become a bottleneck, which is consistent with actual conditions. This also indicates that the terminal compression ratio calculation method is more suitable for high-voltage terminals.
06
Too low a compression ratio will cause excessive temperature rise at the crimp joint. According to actual measurements, when the terminal compression ratio reaches below 40%, the terminal crimp resistance will gradually increase, and the terminal temperature will begin to rise slightly.
07
Analysis of terminal compression ratio applicable to hexagonal crimp terminals.
Firstly, under no circumstances should the cross-sectional area of the crimped joint be less than the nominal conductor cross-sectional area;
Secondly, since the material thickness of each high-voltage terminal will vary, according to the terminal compression ratio = 100T/(At+Ac), some terminals have a lower At cross-sectional area. After compression, an excessively low terminal compression ratio will cause the cross-sectional area at the crimped joint to be smaller than the nominal conductor cross-sectional area.
Therefore, the cross-sectional area after crimping needs to be as large as possible. Furthermore, analysis of tensile data shows that an excessively low compression ratio will lead to a decrease in the mechanical tensile force of the terminal, affecting the reliability of the terminal crimping.
Summarize
Based on the above analysis, considering the reliability of terminal crimping strength and terminal resistance performance, a terminal compression ratio of 65%–75% and a conductor compression ratio of 65%–80% are more suitable.
Experimental data shows that fluctuations in some resistance and temperature rise data are related to the terminal plating and oxidation conditions, as well as the terminal contact structure. Therefore, reducing temperature rise solely based on crimping quality is far from sufficient.
It is necessary to pay attention to factors such as daily light-protected storage of terminals, plating quality, terminal insertion/removal life, insertion force, and contact area. For products with significant temperature rise, high-frequency soldering is recommended. High-frequency soldering melts and bonds the copper through frictional heating at ultra-high frequencies. This method has lower resistance and therefore has a better effect on controlling temperature rise.
